In this study, a flexible skin dosimeter was produced using the PIB method, which is simple to apply to photometric substances and silicone binders. The performance of the skin dosimeter was evaluated.
Fabrication of flexible skin dosimeter
For the lower electrode, a heat-resistant film was applied with indium tin oxide (ITO). The radiation absorption layer was manufactured by mixing 99.99% purity HgO, PbO (Kojundo Chemical Laboratory Inc., Japan) material, and silicone binder at a ratio of 4:1 and applying a screen printing technique to frames of 1 × 1 cm2 and 150 µm thickness. Subsequently, the radiation absorption material was dried for 12 hours at a fixed temperature of 40 °C. The vector placement method was used to create an upper electrode on the top of the radiation absorption material. The upper electrode used gold (Sigma Aldrich Inc. U.S.A.) with 99.999% purity to collect charges. Moreover, the size of the upper electrode was 0.8 × 0.8 cm2.
Figure 6 shows the experimental set-up. The performance of the flexible skin dosimeter was evaluated considering the reproducibility, linearity, dose rate independence, and PDD of the HgO and PbO flexible skin dosimeter at 6 MV and 10 MV. A LINAC system (Infinity; Elekta AB, Stockholm, Sweden) was used for measurements. Considering the 6 MV and 10 MV Dmax photon energy, the build-up materials were set at 1.5 cm and 2.1 cm, respectively. The build-up material used a Slab phantom of equivalent tissue thickness (PTW, RW3, Germany). The source-to-surface distance (SSD) was set to 100 cm. Waveforms were acquired by the oscilloscope to collect signal values from radiation. The charge was calculated from the collected waveforms using ACQ software (Biopac, AcqKnowledge 4.2, CANADA). At this time, a drive voltage of 1 V/μm was applied to the circuit using electrometers (Keithley, 6517A, USA).
Table 1 shows the radiation irradiation conditions used in the experiment.
Table 1. Experimental conditions.
Material
|
HgO, PbO
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Nominal photon energy
|
6 MV, 10 MV
|
Linearity radiation intensity
|
3, 5, 10, 50, 100, 200, 300, 400 MU
|
Reproducible irradiation count
|
10 times
|
Dose rate
|
6 MV
|
100, 300, 430 MU/min
|
10 MV
|
100, 300, 400, 500 MU/min
|
Depth
|
6 MV
|
0.1, 0.3, 0.5, 0.8, 1, 1.3, 1.5, 1.7, 2, 2.5, 3, 5, 10, 15, 20, 25 cm
|
10 MV
|
0.1, 0.3, 0.5, 0.8, 1, 1.5, 1.9, 2.1, 2.3, 2.5, 3, 5, 10, 15, 20, 25 cm
|
Source-to-surface distance
|
100 cm
|
Field size
|
10 × 10 cm2
|
Evaluation
In this study, reproducibility and linearity were measured to evaluate precision and accuracy. In addition, dose rate independence and PDD were evaluated to analyze the response characteristics. Flexible skin dosimeters were irradiated ten times repeatedly for reproducibility measurements. The signals obtained from the first beam were normalized to evaluate the response characteristics of repeated irradiations. Reproducibility assessment can be expressed in RSD based on the amount of the acquired signals. The RSD was calculated as follows:
where Xi and XAve are the measured signal value and mean average signal value, respectively. Moreover, is the number of measurements. The evaluation criteria were set within 1.5% of the RSD value corresponding to the 95% confidence level16–19.
The linearity result was evaluated through the coefficient of determination (R2) of the linear regression that irradiated radiation by gradually increasing the dose in 3, 10, 50, 100, 200, 300, and 400 MU under 50 MU/min conditions. The evaluation criteria for R2 were not less than 0.999012,13. The reproducibility and linearity results were analyzed to evaluate the stability of the signals and the possibility of developing a flexible skin dosimeter.
The dose rate independence was evaluated by increasing the doses gradually by irradiating from 1 to 400 MU under 100, 300, and 430 MU/min at 6 MV and 100, 300, 400, and 500 MU/min at 10 MV. The measured signals were normalized to 200 MU, and the RSD (n=3) for 100 MU measurements was calculated and compared to the diode. The result value of the diode was used in the other study that followed the same experimental method17.
PDD was obtained by increasing the Slab Phantom thickness from 0.1 cm to 25 cm. The results were normalized to calculate the percentage based on the Dmax point and compared with the thimble chamber.